Energy distribution of alkali-metal atoms under electron-stimulated desorption from the surface of oxidized refractory substrates

An analysis is made of experimental data on the dependence of the energy distributions of Li, Na, and Cs atoms emitted in electron-stimulated desorption on their concentration on the surface of oxidized tungsten and molybdenum substrates.     

Samarium atom yield under electron-stimulated desorption from oxidized tungsten surface

The yield of samarium (Sm) atoms under electron stimulated desorption from Sm layers adsorbed on the surface of oxidized tungsten was studied as a function of incident electron energy, surface coverage by samarium, degree of tungsten oxidation, and substrate temperature. The measurements were conducted by the time-of-flight technique with a surface ionization detector in the substrate temperature interval from 140 to 600 K. The yield vs. incident electron energy function has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, which may be related to excitation of the Sm 5p and 5s levels. The Sm yield is a complex function of samarium coverage and substrate temperature. Sm atom peaks occur only in the Sm coverage range from 0 to 0.2 monolayers (ML), in which the yield passes through a maximum. The shape of the yield temperature dependence is a function of Sm coverage. For low Sm cover-ages (<0.07 ML), the yield decreases slowly with the temperature increasing to 270 K, after which it drops to zero at temperatures above 360 K. At higher coverages, the Sm yield passes through a maximum with increasing temperature and additional peaks appear at electron energies of 42, 54, and 84 eV, which can be assigned to excitation of the tungsten 5p and 5s levels. These peaks are most likely associated with desorption of SmO molecules, whose yield reaches a maximum at an Sm coverage of about 1 ML.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

The temperature dependence of Eu atom yield resonances in electron-stimulated desorption from oxidized tungsten

This paper reports on the first measurement of the intensity of the resonances in the yield of europium neutrals as a function of temperature observed in electron-stimulated desorption from tungsten surfaces oxidized to different degrees and having different europium coverages. The measurements were carried out by the time-of-flight method with a surface ionization detector. The temperature dependences obtained for resonances due to europium and tungsten core level ionization differ qualitatively. The relation is reversible for temperatures below the onset of europium thermal desorption.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten

An analysis of the yield q of europium atoms is made, and scenarios of electron-stimulated desorption are put forward. Expressions are obtained for the dependence of q on the coverage of oxidized tungsten by europium atoms.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

Electron-stimulated desorption of sodium atoms from oxidized molybdenum

The time-of-flight technique combined with a surface-ionization-based detector has been used to investigate the yield and energy distribution of sodium atoms escaping in electron-stimulated desorption (ESD) from adlayers on the surface of molybdenum oxidized to various degrees and maintained at T=300 K as functions of incident electron energy and surface coverage by sodium. The sodium-atom ESD threshold is about 25 eV, irrespective of sodium coverage and extent of molybdenum oxidation. Molybdenum covered by an oxygen monolayer exhibits secondary thresholds at ∼40 eV and ∼70 eV, as well as low-energy tailing of the energy distributions, its extent increasing with surface coverage by sodium Θ. The most probable kinetic energies of sodium atoms are about 0.23 eV, irrespective of the degree of molybdenum oxidation and incident electron energy at Θ=0.125, and decrease to 0.17 eV as the coverage grows to Θ=0.75. The results obtained are interpreted within a model of Augerstimulated desorption, in which adsorbed sodium ions are neutralized by Auger electrons appearing as the core holes in the 2sO, 4sMo, and 4pMo levels are filled. It has been found that the appearance of secondary thresholds in ESD of neutrals, as well as the extent of their energy distributions, depend on surface coverage by the adsorbate. Fiz. Tverd. Tela (St. Petersburg) 40, 768–772 (April 1998)     

Temperature and concentration effects in electron-stimulated desorption of sodium atoms from sodium layers adsorbed on tungsten coated with a gold film

The yield and energy distributions of sodium atoms upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a gold film are investigated for the first time as functions of the thickness of the gold film, the concentration of deposited sodium, and the surface temperature. It is found that the energy distributions exhibit two peaks, namely, a narrow peak with a maximum at about 0.15 eV, whose intensity continuously increases with increasing temperature, and a broad peak with a maximum at about 0.35 eV, whose intensity either decreases or remains constant with increasing temperature. It is shown that both peaks arise as a result of the same excitation, which gives rise to different channels of electron-stimulated desorption of sodium atoms. Possible mechanisms of electron-stimulated desorption and the kinetics of destruction of the surface coating are discussed.     

Electron-stimulated desorption of sodium atoms from germanium-coated tungsten

The yield of sodium atoms and energy distribution upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a germanium thin film are measured under variations in the electron energy, the sodium coverage, and the surface temperature by the time-of-flight method with the use of a surface ionization detector. It is revealed that the electron-stimulated desorption of sodium atoms occurs via three channels, namely, a channel involving ionization of adsorbed sodium; the most efficient channel, which is produced by the germanium ionization; and a channel associated with the formation of tungsten excitons, which brings about desorption of NaGe molecules.     

Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten: Concentration dependence of the low-energy peak

The nature of electron-stimulated desorption of europium atoms Eu⁰ at low incident electron energies E _e (～30 eV) and the specific features of the dependence of the yield of europium atoms Eu⁰ on their concentration on the surface of oxidized tungsten are discussed. The crucial stage is found to be the primary event of vacancy creation in the inner 5p shell of the europium adatom. As follows from estimates, only the first of the two possible ionization scenarios (intratomic electron transfer to the outer shell of the Eu adatom or ejection of the knocked-out electron into vacuum) results in Eu⁰ desorption. The concentration threshold of the Eu⁰ yield is determined.     

Cross sections for charge exchange of atoms on multiple-charged ions

Processes of charge exchange of Ar^{+ +}(3p ⁵) and Kr^{+ +}(4p ⁵) on He(1s ²) at low collision energies are investigated theoretically. Semiempirical asymptotic calculations of the radial coupling between initial and final states (both neglecting and allowing for the spin-orbit interaction) are presented. The result of semiempirical calculation of the coupling is in reasonable agreement with an earlier ab initio calculation for the system ArHe^{+ +}. The integral cross sections were estimated using the Landau-Zener model. Comparisons are made with available experimental data and numerical quantum calculation.     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Resonances in the cesium atom yield in electron-stimulated desorption from tungsten coated with a germanium monolayer

The yield and energy distributions of Cs atoms emerging from cesium layers, which are adsorbed on tungsten coated with a thin germanium film (1-to 2-monolayers thick), have been measured as a function of the incident electron energy, the amount of adsorbed cesium, and the substrate temperature. The measurements were performed by the time-of-flight technique with a surface ionization detector. At low cesium coverages (Θ < 0.1), the Cs atom appearance threshold at a substrate temperature T = 160 K is ～24 eV, which correlates with the Cs 5s-level ionization energy. As the electron energy is increased, the yield passes through a broad plateau and reaches saturation. The signal intensity in the plateau region decreases gradually with increasing cesium coverage and tends to zero for Θ > 0.14. For Θ ≥ 0.15, the cesium atom appearance threshold shifts to ～30 eV, which corresponds to the Ge 3d-level ionization energy and the plateau is replaced by a resonance peak at ～38 eV, which can be identified with the ionization energy of the W 5p _3/2 level. This peak is observed only for Θ < 0.3 and T = 160 K. For Θ ≥ 0.3, there appears a resonance peak at ～50 eV, and for Θ ≥ 0.5, another resonance peak appears at ～80 eV. These peak positions correlate with the ionization energies of the W 5p _1/2 and W 5s levels, and their intensity is maximum at Θ = 1. The Cs atom energy distributions for Θ < 0.15 consist of a bell-shaped peak with a maximum at ～0.55 eV, and those for Θ ≥ 0.15 contain two nearly resolved maxima, a broad one peaking at ～0.5 eV and a narrow one at ～0.35 eV. The above results argue for the existence of three channels of Cs atom desorption. One channel involves reverse motion of the Cs²⁺ ion; another channel, neutralization of the adsorbed Cs⁺ ion following the Auger decay of a vacancy in the Ge atom; and the third channel involves desorption of a CsGe molecule as it is repelled from a W core exciton.